It is important to control the bulk density of coal charged into ovens for the production of metallurgical coke used in the iron blast furnace. Bulk density affects proper heating of the coal to produce coke, the level of coal in the oven, the pressure on the walls of by-product coke ovens during the coking process, and the strength of the coke produced. To control the bulk density of a coal blend being prepared for charging into a coke oven, common practice is to measure the bulk density of a coal sample taken from coal on a conveyor belt as it travels from a hammermill, or some other machine designed to pulverize the coal, to coal bunkers for storage prior to charging. The bulk density of the coal is adjusted based on a measured value by adding diesel grade oil, similar oils or other substances in varying amounts to the coal to obtain a desired bulk density value when the coal is subsequently dropped from a known height by a larry car as it is charged into the coke oven. In a manual system, bulk density measurements are made regularly by taking a sample of the coal from the conveyor belt and pouring the sample from a known height into a box of known volume, and then weighing the coal to arrive at a bulk density value in pounds per cubic foot. The flow rate of oil being added to the coal is adjusted manually as required to obtain a predicted post-drop bulk density that is equal to the desired value when the coal is charged into the coke oven.
There are two systems available for automatic control of the bulk density of coal in preparation for the production of metallurgical coke, namely weigh belts and nuclear gamma ray units. The weigh-belt system is relatively expensive to operate and will not be discussed further. With the gamma-ray system, the bulk density of the coal on the conveyor belt is measured as the coal leaves a pulverizer or coal mixer. In both systems the amount of oil or other substance needed to adjust the bulk density is automatically regulated to attain the desired density value. The nuclear gamma-ray measurement system, while complex is capable of performing with an accuracy of plus or minus one pound per cubic foot (i.e. plus or minus 16 kilograms per cubic meter). However, the gamma-ray system is reportedly influenced by a number of factors as described in The Making, Shaping & Treating of Steel, 10th Edition, pages 146-148. Some of the most important factors reported in the reference are: (1) the depth of the coal on the belt as it passes under the radioactive source; (2) changes in the radiation-absorption coefficient of the coal; (3) dust or other material in the signal path; (4) temperature of the detector; (5) thickness and tension of the belt; and (6) size consistency, moisture content and temperature of the coal. While moisture content has been generally recognized as a factor in the measurement of bulk density using gamma radiation, it generally is believed that the effect is small based on the difference in the absorption coefficient of gamma radiation by water and coal.
U.S. Pat. No. 3,678,268 to Reim et al, discloses a gamma radiation bulk density gauge for measuring the bulk density of coal on a conveyor belt and a system for controlling the bulk density by varying water and oil addition rates based on the bulk density measurements. The rate of water addition is controlled to bring the rate of oil addition within a desired range. Prior to leveling the coal on a second conveyor and measuring the bulk density, the coal is dropped from a first conveyor onto the second conveyor. The height of the first conveyor is adjusted to make the drop substantially the same as the drop the coal undergoes in the coke oven. Where the conveyor system cannot be adjusted to simulate the drop into the oven, alternate devices are described for simulating the drop, e.g. the sled and weights shown in FIGS. 6-8 and the paddle wheel assembly of FIG. 9 of the reference. This reference does not disclose prediction of the post-drop coal bulk density by the measurement of the coal bulk density and moisture content prior to drop, for example while the coal is on a conveyor belt, nor the correlation of such measured bulk density and moisture content with the predicted post-drop bulk density. R. H. Lux and A. D. Strauss, 1996 Ironmaking Conference Proceedings, pages 515-520 also describe another automated bulk density control system in which gamma radiation bulk density gauges are used.
A gauge for continuously measuring the moisture content of coal of varying thickness on a conveyor belt using microwaves is disclosed in U.S. Pat. No. 5,333,493 to Cutmore. A gauge that measures both moisture content and bulk density of coal is described in U.S. Pat. No. 4,766,319. This gauge uses a low activity neutron radiation source to measure moisture and a low activity gamma radiation source to measure bulk density. A correction signal is applied to the density measurement based on the moisture content to account for gamma radiation produced by a thermal neutron capture reaction which occurs when neutron radiation is used to measure moisture content of the coal. The reference does not suggest prediction of post-drop bulk density based on a correlation of bulk density and moisture content measurements made while the particulates are at rest before drop, particularly while the particulates are at rest on a conveyor belt.
Other miscellaneous references are: U.S. Pat. Nos. 4,304,636; 4,450,046; 4,506,541 and 5,435,541.